Flywheel energy storage has garnered some interest from academia and industry for its potential to store surplus electrical energy efficiently in kinetic form.
Modern designs use magnetic bearings to minimize the drag that the rotating mass incurs by levitating it in its entirety within a vacuum chamber. Most serious research efforts seem to implement these bearings with superconducting magnets cooled to 50 K or lower, in order to take advantage of a phenomenon called flux pinning that apparently occurs under these conditions.
This flux pinning stabilizes the flywheel in a way that room temperature permanent ferromagnets alone (being a collection of point charges) are not able to, due to Earnshaw's theorem.
However, there also exist materials such as bismuth and pyrolytic carbon, which even at room temperature exert diamagnetic forces quite capable of stabilizing objects that are magnetically levitated by permanent ferromagnets.
Why not use these diamagnetic materials instead of the superconducting variety, and greatly reduce the complexity, cost and refrigeration losses of the flywheel design?
One possible reason for using superconductors could be that flux pinning might suffer less from eddy currents ("electromagnetic drag") than room temperature diamagnets, but I'm not sure how to evaluate the impact of this effect, if any; so an answer that attempts to shed some light on this aspect would be much appreciated.